In numerous electrochemical devices, some metallic components need a low electrical surface contact resistance with adjacent components. The metallic components could be electrodes, current collectors, mass (gas or liquid) diffusion layers, or the multifunction components, such as bipolar plates in fuel cells and electrolysis cells. The metallic components must also be corrosion resistant in the operational environment to ensure long term, stable operation. However, most corrosion resistance metals and alloys rely on a surface oxide layer for corrosion protection, and the surface oxide layer is not electrically conductive.
One example application is the metallic components in electrolysis cells. An electrolyzer is an electrochemical device that performs chemical reactions using electrical power. The typical reactions include splitting water to hydrogen and oxygen, or converting sodium chloride to chlorine gas and sodium hydroxide. A practical electrolyzer includes multiple cells for sufficient production capacity. These cells are connected in series using bipolar plates to build a stack with other necessary hardware such as gas diffusion layers, end plates, cell frames, gaskets, etc. These components are exposed to high electrochemical potential during operation. Metallic components, typically made of commercial pure titanium, will be continually oxidized resulting in a thicker surface oxide layer and higher electrical contact resistance. The high surface electrical contact resistance will lead to the high internal energy loss of the electrolysis cells. The current solution is to electrically plate the component with a layer of platinum to ensure low surface electrical contact resistance. The high material cost of platinum prohibits the broader commercial applications.
Another example application is the current collector of a lithium battery. The function of the current collector is to collect electrons to and from electrode active materials to maintain the battery's operation. It is desired to have low surface electrical contact resistance in high voltage operational conditions. Commercial 3.6V lithium ion batteries based on the LiPF6 electrolyte use a high purity aluminum foil as the current collector. It works fine in small to medium power batteries. However, both electrical current density and operating voltage of high power lithium batteries are much higher than conventional applications. The internal ohmic loss is high (under high electrical current) and the aluminum is not stable (under high voltage). More surface conductive and corrosion resistant current collector material is needed for high power lithium batteries.
One further example is the bipolar plates in proton exchange membrane (PEM) fuel cell stacks. The function of the bipolar separate plate is to 1) separate the hydrogen of one cell from the air of the adjacent cell; 2) collect electrons to/from the electrodes; 3) facilitate mass (gas and water) transport in cells and 4) maintain the proper operational temperature of the stack. The operational condition inside the PEM fuel cell is very corrosive (high potential, in acid solutions). The electrical current density is very high (2-4 A/cm2). Bipolar plates made of metal foil, such as stainless steel foil, have advantages such as light weight and higher thermal conductance than that of traditional graphite plates. However, it is a challenge to maintain low electrical contact resistance of a metal plate with a gas diffusion layer (the component in direct contact with bipolar plates), due to the resistivity of the surface oxide layer of corrosion resistant metal plates.
The electrical contact resistance and corrosion resistance requirements are dependent on the specific applications. For example, the metallic components in PEM electrolysis cell must have a corrosion resistance that is suitable for over 10 years of stable operation in a slightly acidic solution at high electrochemical potential (over 2.0 VNHE) with electrical contact resistance below 2 mΩ·cm2 with a porous titanium gas diffusion layer. In PEM fuel cells, the metallic bipolar plates must have sufficient corrosion resistance for stable operation over 6,000 hours in an acid solution under an electrical potential over 0.8VNHE at 80-90° C. The surface electrical contact resistance must be below 10 mΩ·cm2 with a carbon felt gas diffusion layer. The current collector for lithium batteries has to be stable in the organic salt electrolyte solutions (typically containing fluorine) at high potential (up to 5 VLi/Li+). The surface electrical contact resistance of the current collector with electrode should be below 100 mΩ·cm2. The requirements for a capacitor current collector are also related to the chemical systems (aqueous or organic electrolyte based) and the operational current density of super-capacitors are at a much high current density than that of traditional capacitors. Therefore, lower resistant current collectors are desired.
In electrochemical devices, metallic components, as a solid piece, are in contact with other solid components, in most cases, under certain compression pressure. It is well known that the solid-to-solid electrical contact is built on the direct contact of high points of the solid surface. The common way to reduce the electrical contact resistance is to use soft materials, such as gold, silver, tin and copper, that can be deformed under the pressure to increase the contact area between solid components. An example of this type of surface modification is taught in U.S. Pat. No. 6,685,988, which deposits tin on a metal surface to reduce the electrical contact resistance. However, these soft materials are either too expansive or lack the desired corrosion resistance for electrochemical applications.
Various methods have been taught for these applications. U.S. Pat. No. 6,379,476 teaches a special stainless steel alloy that has electrical conductive inclusions for PEM fuel cell applications. The surface oxide layer of the special alloy can protect the stainless steel from corrosion in PEM fuel cells, and the electrical conductive inclusions can maintain the low surface electrical contact resistance. US patent application publication no. 2005/0089742 teaches a method to etch off the surface metallic layer of the alloy similar to the one taught in U.S. Pat. No. 6,379,476 to expose the electrical conductive inclusions as the electrical contact point.
U.S. Pat. No. 6,723,462 no. teaches a special chromium-nickel austenitic alloy that can form a more electrical conductive surface oxide than regular stainless steel, such as commercial available 316L. It can be used as the bipolar plates for PEM fuel cells.
U.S. Pat. No. 5,098,485 teaches a method to convert the electrical insulating, native poorly conductive surface oxide layer of metallic components to more electrically conductive surface for capacitor applications.
WO 2007/013508 teaches a Ti alloy with precious metal elements. The precious metal in the alloy will lead to the formation of a precious metal and titanium oxide composite surface layer that has low surface electrical resistance. It can be used as an electrode or bipolar plates in fuel cells.
In addition to the development of special alloys that have low electrically resistive surface oxide layer, another widely used approach is to coat the metallic components with corrosion resistant and surface electrical conductive materials, typically precious metals. US patent application publication no. 2003/0124427 teaches a method to deposit a thin layer of gold on titanium plate by sputtering process. The gold coated titanium plate is used as the current collector in nonaqueous Li/CFx cells. Platinum coated titanium has been widely used in electrolyzers. Gold coated stainless steel plates were also used for PEM fuel cell applications. In general, the corrosion resistance and electrical conductive coating on a low cost substrate material is a practical approach for various applications, as long as the coating material has reasonable cost, and the coating process is reliable and low cost.
Neil Aukland reported a group of low surface electrical contact resistance titanium alloys that contain 1-3 atomic % niobium or tantalum (J. Mater. Res., Vol. 19, No. 6, pp. 1723-1729, June 2004). This alloy can form a semi-conductive niobium doped titanium oxide (or tantalum doped titanium oxide) surface layer that leads to lower surface electrical contact resistance than pure titanium. This method has the advantage of low metal cost by eliminating precious metal. However, its surface electrical contact resistance is still too high for PEM fuel cell applications.
Therefore, it is desired to have metallic components that have high corrosion resistance and low surface electrical contact resistance for electrochemical applications. It could be either a special alloy or a coating on a metallic components. Methods for the alloy and the coating production and surface treatment are also needed to produce low cost, durable metallic components for electrochemical applications.